1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for performing fine frequency offset compensation to achieve frequency synchronization.
2. Description of the Related Art
In general, phase jitter and Doppler shift as a natural occurance of radio channels. Also, asynchronization between a transmit frequency and a receive frequency arises from instability of a receiver tuner in a mobile communication system. Each of these cause a frequency offset. The transmit frequency is the frequency of a carrier that delivers signals in a transmitter, whereas the receive frequency is the frequency of a carrier by which a receiver receives signals. The tuner instability relates to a phenomenon where tuning is not realized between a transmitter local oscillator and a receiver local oscillator, that is, between the carriers.
FIG. 1 conceptually illustrates transmitting and receiving apparatuses in a typical QPSK (Quadrature Phase Shift Keying) mobile communication system. As illustrated in FIG. 1, the transmitting apparatus transmits information in an in-phase (I channel) signal and a quadrature-phase (Q channel) signal, which have a 90-degree phase difference. The receiving apparatus receives the I and Q channel signals and demodulates desired information from the received signals.
Referring to FIG. 1, a transmitter 110 upconverts input baseband I and Q channel signals to RF (Radio Frequency) signals by a predetermined carrier frequency. A power amplifier (PA) 120 amplifies the RF signals to a predetermined transmit power level and transmits them through an antenna. The RF signals are received at the receiving apparatus through an antenna. A low noise amplifier (LNA) 130 amplifies the received RF signals such that noise is reduced and the strength of an original signal is increased. A receiver 140 downconverts the RF signals received from the LNA 130 to baseband signals. For the downconversion, the receiver 140 must use the same carrier frequency as used in the transmitter 110. However, it is impossible to upconvert or downconvert the signals using the same carrier frequency in the transmitter 110 and the receiver 140 because of the afore-mentioned frequency offset. Therefore, compensation for the frequency offset is essential to the increase of downconversion reliability in the receiver 140.
FIGS. 2, 3 and 4 illustrate implementation examples of the receiver illustrated in FIG. 1.
FIG. 2 illustrates a receiver in which I and Q channel signals are downconverted by means of a single mixer. In FIG. 2, an input signal (i.e. “RF”) “A cos ωRFt+B sin ωRFt” is downconverted by “cos (ωLO−Δω)t” and “sin (ωLO−Δω)t”, respectively and I and Q channel signals are thus produced.
FIG. 3 illustrates a receiver in which I and Q channel signals are downconverted twice by means of two mixers.
Referring to FIG. 3, the input signal (i.e. “RF”) “A cos ωRFt+B sin ωRFt” is primarily downconverted by first multiplying it by a first carrier “cos (ωLO1−Δω1)t”. The primary downconversion signal (i.e. “IF”) is downconverted by second carriers “cos (ωLO2−Δω2)t” and “sin (ωLO2−Δω2)t. The carrier ωRFt used in a transmitter is defined as “ωLO1+ωLO2” and the frequency offset Δω is defined as “ω1+Δω2”. The signal downconverted by the second carrier cos (ωLO2−Δω2)t is output as a I channel signal, whereas the signal downconverted by the second carrier sin (ωLO2−Δω2)t is output as a Q channel signal.
FIG. 4 illustrates another receiver in which I and Q channel signals are downconverted twice by means of two mixers.
Referring to FIG. 4, the input signal. (i.e. “RF”) “cos ωRFt+B sin ωRFt” is primarily downconverted by multiplying it by first carriers “cos (ωLO1−Δω1)t” and “sin (ωLO1−Δω1)t”. Each of the primary downconversion signals is downconverted by second carriers “cos (ωLO2−Δω2)t” and “sin (ωLO2−Δω2)t”. A signal downconverted by the first carrier “sin (ωLO1−Δω1)t” is downconverted by the second carriers “cos (ωLO2−Δω2)t” and “sin (ωLO2−Δω2)t”. The carrier ωRFt used in the transmitter is defined as “ωLO1+ωLO2” and the frequency offset Δω is defined as “ω1+Δω2”. The signal is downconverted by the first carrier “sin (ωLO1−Δω1)t” and then downconverted by the second carrier “sin (ωLO2−Δω2)t”. This signal is subtracted from the signal downconverted by the first carrier “cos (ωLO1−Δω1)t” and then downconverted by the second carrier “cos (ωLO2−Δω2)t”. The difference signal is converted to a digital signal and output as an I channel signal. Meanwhile, the signal downconverted by the first carrier “cos (ωLO1−Δω1)t” and then downconverted by the second carrier “sin (ωLO2−Δω2)t” is added to the signal downconverted by the first carrier “sin (ωLO1−Δω1)t” and then downconverted by the second carrier “cos (ωLO2−Δω2)t”. The sum signal is converted to a digital signal and output as a Q channel signal.
The I and Q channel signals output from the receivers illustrated in FIGS. 2, 3 and 4 are not ideal signals including only an amplitude component A or B. They also include frequency components due to the frequency offset Δω. In other words, all of the carrier frequency components are not eliminated during the primary and secondary downconversions because of the frequency offset Δω.
This frequency offset changes the phase of the input signal. The resulting loss of orthogonality between the carrier frequencies degrades decoding performance in the system. However minimal the frequency offset is, the frequency offset is a critical factor that degrades the performance of a receiver system. FIG. 5 illustrates simulated received symbol distributions under varying frequency offsets. The frequency offsets illustrated are 0.2, 1, and 10 ppm(pulse position modulation). As noted from FIG. 5, the error rate of received symbols increases with the frequency offset. Therefore, frequency synchronization techniques are essential to prevent the loss of orthogonality between carriers caused by a frequency offset.
One method for correct for the frequency offset is based on the interval between carrier frequencies in the receiver. Being divided by the carrier frequency interval, the frequency offset can be expressed as an integer part and a fraction part. The process of eliminating an initial frequency offset corresponding to the integer part is referred to as coarse frequency synchronization and the process of eliminating a frequency offset corresponding to the fraction part, that is, the residual frequency offset after the coarse frequency synchronization, is referred to as fine frequency synchronization. Frequency synchronization techniques in an OFDM (Orthogonal Frequency Division Multiplexing) mobile communication system are categorized into algorithms using time-domain signals before FFT (Fast Fourier Transform) and algorithms using frequency-domain signals after the FFT. In the former algorithms, a long preamble additionally transmitted together with data is used to compensate for the frequency offset, as proposed in the IEEE 802.11a WLAN (Local Area Network) standards.
As described above, the conventional mobile communication system transmits a long preamble by which the frequency offset is estimated and compensated for. In other words, a transmission frame includes a long preamble and a complex algorithm is used to compensate for the frequency offset by the long preamble in the conventional frequency offset compensation.